Signal interrupting means for audiometers



Feb. 9, 1943. M. MAGEs SIGNAL INTERRUPTING MEANS FOR AUDIOMTERS FiledJan. 26. 1942 Atient hears the tones.

Patented Feb.. w43

SIGNAL INTERRUPTING' MEAN S FOR AUDIOMIETEBS Morris Magies, Chicago,Ill., assignor to V. Mueller & Company, Chicago, Ill., a corporation ofIllinois Application-January 26, 1942, serial No. 428,326

6 Claims.

This invention particularly relates to the noiseless interruption andnoiseless restoration of the signal or sound output of an audiometer. Inan audiometer the method of testing consists of generating test tones ofvarious frequencies and determining the loudness level at which the pa-In such testingr it is very desirable for'the operator of the audiometerto f be able to cut out the tone and restore it again noiselessly andwithout any click or other extraneous noises, as a check in determiningwhether the patient hears or does not hear the test tone at a givenloudness level. The absence of any click in the switching process isnecessary to avoid confusing the patient and to prevent a patient fromguessing as to when the tone is on or off by means of the click. In somecases the click may be pronounced enough to startle the patient orrender the testing process uncomfortable.

It has been found lthrough experience that a click is bound to arisefrom switching whenever the change from a high level to a very low levelis made very abruptly as in ordinary switching. It appears that thesteep wave front which results from such a process necessarily generatesa series of high frequencies which is heard as a click. The onlysatisfactory method of avoiding a click is to make the transitiongradual so as to result in a fading out or in of the tone. The methodfor accomplishing this satisfactorily is the subject of. this invention.

A brief description of the method follows. 'By means of a push button ahigh negative bias is applied to lone of the tubes of the audiometertransmitting the signal, this negative bias voltage being automaticallyand gradually transmitted to the` grid of the vcontrolled tube by thedelaying action of a condenser-resistance network. Conversely, when theproper operating grid voltage is restored by releasing the push button,the bias voltage between grid and cathode gradually assumes its normalvalue as the resistor-condenser network discharges, and the test tone isrestored to full intensity.

The method is. not limited to the control of one tube. Two or more tubesmay be simultaneously controlled by arranging the condenser resistornetworks and the circuit for the push button as shown in the drawing.Other modifications of the circuit such as the exact arrangement of thepush button circuit; and modifications of the resistor condensernetwork, will be apparent to one skilled in the art.

Fig. 1 shows the method applied to a single tube placed between thesource of signal voltage and the output device; Fig. 2 shows the methodapplied to -two tubes v to make theaction more positive; and

Fig. 3 shows a modification of the push button circuit.

Referring to Fig. 1, I indicates a triode tube functioning as a transfertube or output tube in an audiometer. The elements of the tube are aplate 2, a grid 3, and a cathode d. A filament 5 for heating the cathodeis supplied from a transformer or other suitable source. furnishes thenormal grid bias voltage. An interrupter push button I shunts theresistor 3.

The push button 'I is in ya normally closed position for normaltransmission of the signal through the tube I. An electrolytic condenser9 by-passes the resistors 6 and 8 to prevent ripple voltage from the Bsupply from being transmitted to the grid of the tube. A resistor I0normally supplies a portion of the current through the resistor 6. Whenthe push button I is operated current flows through both of theresistors 8 and 6 which results in a high voltage being set up betweenthe cathode and B-. The elements II,

I2, I3 and Ili form a two section resistor-con denser network whichfurnishes the time delay action in response to the action of the pushbutton l. A resistor I5 and a condenser I6 are the usual grid leak andcoupling condenser. The source of signal Voltage I'l may be the platecircuit of another amplifier tube or the output terminals of anoscillator. The output of the tube I is transmitted through atransformer I8, through an attenuator I9, and then to an output device2t. The source of plate voltage for the tube I is indicated by B'- andB+. The attenuator I9 may be that shown in my co--pending application,Serial blo-428,325, filed Jan. 26, 1942.

In Fig. 2 a, more complete controlgircuit is shown wherein an additionaltube 2l which ampliies the signal voltage I'I is also subject to thefade out control by the push button 1. All of the elements identical toFig. 1 have the same An element 22 denotesv A resistor 6 resistor 29 isofa value to give the proper operatingl grid bias for the tube. Aresistor 30 is high in resistance compared to the resistor 29. Operatingthe' bush'button 'I causes the resistor 30 to be substituted for theresistor 29 as a bias resistor thereby creadting a high bias voltagebetween the grid and the cathode of the tube.

Referring to the operation of the circuit shown in Fig. 1, the signalvoltage Il is applied to the grid and cathode of the tube through thecoupling condenser IB. 'I'he actual grid bias is the voltage across thecondenser I3. It is evident that after a condition of equilibrium hasbeen reached this voltage will be equal to the voltage drop across theresistor 6 which is determined by the plate current of the tube I andthe bleeder resistor I0. The tube I, operating at normal grid bias,ampliiles the signal voltage and it is transferred to the outputtransformer, through the attenuator and iinally to the output device. Inorder to cause the signal to fade to inaudibility the push button 1 ispressed down and held open. This brings the resistor 8 into the circuitwith the result that a high D. C. voltage is set up across the resistor8 in series with the resistor 6. The cathode end becomes more highlypositive with respect to the B- terminal. The

change in voltage, however, takes a definite time V to reach the grid ofthe tube as rst the condenser II charges up through the resistance I2.As the voltage across the condenser I3 builds up,

' making the grid more negative with respect to the cathode, the platecurrent of the tube I is gradually brought to cut-oi and there is notransfer of the signal voltage through the tube. Conversely, when thepush button is released the condenser II discharges through theresistance I2 and then the condenser I3 discharges through theresistance I4, giving a gradual build up of the output signal to itsnormal or preset level.

In this manner all trace of any click in the output is avoided either atthe on" or off position of the push button. Typical resistance andcondenser values for a triode tube are: resistor 6=700 ohms, resistor8=20,000 ohms, condenser 9=10 mf., resistor 10=250,000 ohms, condenser11:8 mf., condenser 13:4 mf., resistor 12:50,- 000 ohms, and resistor14=50,000 ohms.

In Fig. 2 two tubes are shown controlled by the push button 1. It may bedesirable to do this if suiicient cut-off is not obtained by the tube Idue to transfer of the signal through interelectrode capacities. Thefunctioning of the tube 2 is the same as tube I. may be made the same asfor tube I or a tap may be taken oii the resistor 8 and the elements 23and 24 omitted.

In Fig. 3 the resistors furnishing the normal bias voltage and cut-oibias voltage are in par'- allel instead of in series.

Other modifications of the circuit-l may be made in the resistorcapacity network such as changing the number of sections or altering theform of the network in such manner as would occur to one skilled in theart.

It must be understood that I do not limit myself to the particular typeof tube shown, the particular type of resistance condenser network,

or the push button circuits shown in this applica- Its normal bias ergyof said frequency, means for causing a graddensers acting gradually totransmit the voltage change made by said bias changing means to thegrid. y 1

3. In an audiometer having a load device adapted to produce audibletones of a given frequency in\response to an input of electrical en`ergy of said frequency, means for causing a gradual interruption orrestoration of the tones, said means including a vacuum tube interposedbetween the source of energy and the load device, said tube having acathode, a plate and a grid, means operable to cause a change inappliedbias voltage to said grid, and time delay means interposedbetween the bias changing means and the grid including a network ofresistors and condensers acting gradually to transmit the voltage changemade by said bias changing means to the grid, said bias changing meanscomprising a series of resistors interposed between the positive andnegative voltage source leads for said tube.

y and a switch shunting one of said resistors.

4. In an audiometer having a load device adapted to produce audibletones of'a given Irequency lin response to an input of electrical energyof said frequency, means for causing a gradual interruption orrestoration of the tones, said means including a vacuum tube interposedbetween the source of energy and the load device, said tube having acathode, a plate and a grid, means operable to cause a change in appliedbias voltage to said grid, and time delay means interposed between thebias changing means and the grid automatically responsive to operationof said bias changing and acting gradually to transmit thevoltage'change made by said bias changing tion. The method may even beapplied to some other element than the control grid of a tube as forinstance the screen grid or suppressor grid of a vacuum tube.

Having thus described my invention, what I means to the grid.

5. Apparatus for causing a gradual interruption or restoration of asignal voltage being transmitted through a vacuum tube interposedbetween a source of energy and a load device, said tube having acathode, a plate and a grid, means operable to cause a change in appliedbias voltage to said grid, and time delay means interposed between thebias changing means and the grid including a network of resistors andcondensers acting gradually to transmit the voltage change made by saidbias changing means to the grid, said bias changing means comprising aseries of resistors interposed between the positive and negative voltagesource leads for said tube. two of said resistors forming the grid biasfor said tube, and a switch shunting one of said bias re-` sistors, twoof the resistors in said series being cathode, and one end of thecondenser being con-y in parallel and said switch being in series'withonly one of said two resistors.

6. Apparatus for causing a gradual interruption or restoration of asignal voltage being transmitted through a. vacuum tube interposedbetween a source of energy and a load device, said tube having acathode, a plate and a grid, means operable to cause a change in appliedbias voltage to said grid, and time delay means interposed between thebias changing means and the grid including a network of resistors andcondensers acting gradually to transmit the voltage change made by saidbias changing means to the grid, said network consisting 'of at leastone section made up of a resistor and condenser in series, one end ofthe resistor being connected to a point of negativevoltage with respectto the nected tothe cathode, a second section oi.' resis-1 tor andcondenser connected in shunt with the condenser of the rst section, thecondenser of the second section forming the connecting element betweenthe cathode and lower end o! the grid return path of the above mentionedtube, also bias changing means comprising a series of resistorsinterposed between `the positive and negative voltage source leads forsaid tube, two

of said resistors forming the grid bias for saidl tube, and a switchshunting one of said bias resistors, two of the resistors in said seriesbeing in parallel and said' switch being in series with only one of saidtwo resistors.

MORRIS MAGEs.

